A prospective study of 55 orthognathic surgical patients was done to determine the effects of surgery on mandibular range of motion. None of the patients had oral physiotherapy during the course of the study. Nineteen patients had mandibular osteotomies, 21 had maxillary osteotomies, and 18 had two-jaw operations. Maximal interincisal opening (MIO), right and left lateral excursion, and protrusive measurements were obtained preoperatively and at six or more months following surgery. MIO was significantly reduced in both categories of mandibular osteotomies. A sagittal split osteotomy to advance the mandible was associated with the greatest mean reduction of 29%, while a vertical subcondylar osteotomy to set the mandible back had a mean reduction of 10%. Likewise, decreases in MIO were noted with combined surgical procedures. Le Fort I and sagittal split osteotomies were associated with a mean decrease in MIO of 28%, while Le Fort I and vertical subcondylar osteotomies had a mean decrease of 9%. Minimal change in MIO were noted with isolated maxillary osteotomies. These results are similar to the findings of other investigators and indicate the critical need for a sound postoperative rehabilitation program following orthognathic procedures to prevent hypomobility.